1. Field of the Invention
The present invention is in the field of medical measuring technology. In particular, the present invention relates to methods and apparatuses for detecting sleep apnea from cardiological data.
2. Description of the Related Art
Breathing pauses which last longer than 10 seconds are referred to as apnea. Breathing pauses of this type also sometimes occur in healthy people during sleep, this is physiologically normal sleeping behavior. In contrast, “sleep apnea syndrome” as a diagnosis is understood as the cyclic occurrence of respiratory arrest (of over 10 seconds duration) during the nocturnal sleeping phase having a repetition rate of more than 5 apneas per hour. The repetition rate is a measure of the degree of severity of the sleep apnea and is referred to as the apnea index. An apnea index of 5-10 corresponds to a light sleep apnea, 10-20 to a moderate sleep apnea, and more than 20 to a severe sleep apnea. A sleep apnea always results in significant health impairment (inter alia, heart rhythm interference, higher blood pressure, depression caused by sleep interruption), in some cases, death may even occur. It is estimated that approximately 3-5% of the adult population in the Western industrial nations are affected by sleep apnea. While the so-called obstructive form of sleep apnea occurs very frequently, particularly patients having congestive heart failure (CHF) are also affected by the central form (approximately 10% of all cases of sleep apnea, >30% in the event of CHF). However, mixed forms are also very common.
The occurrence and termination of a sleep apnea (obstructive or central) are characterized by a complex interplay of different factors. The normal sleeping state already differs in many ways from the waking state and in turn is composed of an array of different stages. These are differentiated primarily according to REM sleep (25%) and non-REM sleep (REM=rapid eye movement). Non-REM sleep is in turn composed of four different stages and all five classes are characterized, inter alia, by differing brain activity, which is expressed in various EEG patterns. An important special feature of REM sleep is atonia, which results in relaxation of the skeletal muscles.
The muscle atonia of REM sleep favors the occurrence of an obstruction, i.e., a temporary blockage of the airways, by relaxation of the pharyngeal muscles. If this occurs, the air is entirely or partially obstructed from flowing into the lungs in spite of existing respiratory movements. The direct result of this is a measurable reduction of the oxygen saturation of the blood. Selective chemoreceptors are stimulated by the drop of the oxygen saturation, which results in a nearly linear rise of the sympathicotonia over time. This continues for a period of time from at least 10 seconds to over 40 seconds. The increasing sympathetic activation results at its end in an activation of the brain activity up to leaving the sleep phase (which the patient usually no longer remembers in the morning). This results in a temporary removal of the obstruction, which may again occur after a period of time of 10 seconds to 40 seconds, however.
In the central form of sleep apnea, there is a breakdown of the control signals of the brain relevant for the breathing action, so that the respiratory movements do not occur at all. However, the result is also a cyclic cessation and resumption of the oxygen supply with a corresponding effect on the sympathicotonia.
The entire procedure during sleep apnea is thus not static, but rather comprises a dynamic oscillation. The cyclic activations occurring again and again are suspected to result in damage in the area of blood pressure regulation and the cardiac muscle, but are certainly disadvantageous because of their effect on the overall regulation by the autonomous nervous system. Important clinical symptoms of patients having sleep apnea are general sleepiness as well as a sympathicotonia elevated in relation to healthy people measurable via the muscular sympathetic nerve activity even in the waking state, this tonicity also not having the drop in the nocturnal phase which is typical in healthy people. The diurnal tonicity increase may also be confirmed in controlled experiments using analysis of the heart rate variability (HRV).
The standard method for diagnosing a sleep apnea comprises performing a polysomnographic examination in a suitable laboratory. For this purpose, there is long-term monitoring of the patient by an external Holter device, which is essentially based on measurement of the surface ECG and may optionally comprise further sensors, such as pulse oximetry probes.
This method has several disadvantages. Thus, it is certainly not stress-free for the patients, i.e., there are significant concerns in relation to acceptance and compliance in regard to long-term use of Holter devices by the patients. The analysis of the long-term measurements requires correspondingly high-performance data storage and processing and may only be automated to a restricted extent. Overall, the outlay required is significant, so that such examinations are costly. Long-term Holter examinations are therefore preferably first performed when there is already well-founded suspicion of apnea and obviously also may not be repeated at arbitrary intervals.